Laminography techniques are widely used to produce cross-sectional images of selected planes within objects. Conventional laminography requires a coordinated motion of any two of three main components of a laminography system (i.e., a radiation source, an object being inspected, and a detector). The coordinated motion of the two components can be in any of a variety of patterns, including linear, circular, elliptical and random patterns. Regardless of the pattern of coordinated motion selected, the configuration of the source, object and detector is such that any point in the object plane (i.e., the focal plane within the object) is always projected to the same point in the image plane (i.e., the plane of the detector), and any point outside the object plane is projected to a plurality of points in the image plane during a cycle of the pattern motion. In this manner, a cross-sectional image of the desired plane within the object is formed on the detector. The images of other planes within the object experience movement with respect to the detector, thus creating a blur, i.e. background, on the detector upon which the sharp cross-sectional image of the focal plane within the object is superimposed. This technique results in sharp images of the desired object focal plane. Although any pattern of coordinated motion can be used, circular patterns generally are preferred because they are more easily produced.
The laminography techniques described above are currently used in a wide range of applications including medical and industrial x-ray imaging. Laminography is particularly well suited for inspecting objects that comprise several layers, with each layer having distinguishable features. However, laminography systems that produce such cross-sectional images typically experience shortcomings in resolution and/or speed of inspection, thus accounting for the rare implementation of laminography systems for this purpose. These shortcomings are frequently due to the difficulties in achieving high speed coordinated motion of the source and detector to a degree of precision sufficient to produce a high resolution cross-section image.
In a laminography system having a field of view that is smaller than the object being inspected, it may be necessary to move the object around within the field of view to obtain multiple laminographs which, when pieced together, cover the entire object. Movement of the object is frequently achieved by supporting the object on a mechanical handling system, such as an X, Y, Z positioning table. The table is then moved to bring the desired portions of the object into the field of view. Movement in the X and Y directions locates the area to be examined, while movement in the Z directions moves the object up and down to select the plane within the object where the image is to be taken. While this method effectively enables various areas and planes of the object to be viewed, there are inherent limitations associated with the speed and accuracy of such mechanical motions. These constraints have the effect of increasing cycle time, thereby reducing the rates at which inspection can occur. Furthermore, these mechanical motions produce vibrations that tend to reduce the system resolution and accuracy.
In order to reduce or eliminate the need to move the object, and the problems associated therewith, an off-axis laminography system has been invented, which is disclosed in U.S. Pat. No. 5,259,012 (the '012 patent) and which is incorporated herein by reference in its entirety. The '012 patent discloses a laminography system in which off-axis scanning circles can be used to enable multiple locations on an object to be sequentially imaged without requiring mechanical movement of the object or of the electron beam gun that is used to generate the x-rays. The phrase “off-axis” refers to placing the center of the scan circle in a position that is not concentric with the optical axis of the imaging system. The electron beams are projected from the gun onto a metal target anode. When the electron beams impinge on the target anode, x-rays are produced. The electron beams are deflected by a voltage-controlled yoke that causes the electron beams to impinge on the target anode at selected locations to trace off-axis circles that enable different locations on the object to be scanned.
FIG. 1 illustrates a schematic diagram of a laminography system 10 disclosed in the '012 patent. The system 10 comprises a source of x-rays 12 positioned above an object 14 to be imaged, and a rotating x-ray detector 16, positioned below the object 14 and opposite the x-ray source 12. The object 14 may be, for example, a printed circuit board, a manufactured item such as, for example, an aircraft part, a portion of a human body, etc. The system 10 is symmetrical about a Z-axis 50. The system 10 acquires X, Y plane cross-sectional images of the object 14 under inspection using multi-path laminography geometries, which enable multiple locations of the object 14 to be sequentially imaged without requiring mechanical movement of the object 14. In other words, off-axis (i.e., not about the axis 50, but about an axis parallel to axis 50) scanning patterns are used to image the object over different regions of the object in the X, Y plane.
The laminography system 10 may be interfaced with an analysis system 15 that automatically evaluates the cross-sectional image generated by the system 10 and provides a report to a user indicating the results of the evaluation. The source 12 is positioned adjacent the object 14, and comprises an electron gun 18, a set of electrodes 20 for electron beam acceleration and focus, a focus coil 60, a steering yoke or deflection coil 62, and a substantially flat target anode 24. An electron beam 30 emitted from the electron gun 18 along the Z-axis 50 is incident upon the target anode 24 and causes an x-ray spot 32 to be produced, which serves as an approximate point source of x-rays 34. The x-rays 34 emanate from a point on the target anode 24 where the electron beam 30 impinges upon the target anode 24. At least a portion of these x-rays pass through various regions of the object 14 and impinge on the detector 16.
The object 14 is mounted on a platform 48 which may be affixed to, for example, a granite table 49, so as to provide a rigid, vibration-free platform for structurally integrating the functional elements of the system 10, including the x-ray source 12 and the turntable 46. It is also possible that the platform 48 comprises a positioning table that is capable of moving the object 14 along three mutually perpendicular axes; labeled X, Y, and Z in FIG. 1. As stated above, with off-axis scanning, it is not necessary to physically move the object 14. However, it may be desirable to move the object 14 to some degree to improve image quality. At any rate, with off-axis scanning, it is not necessary to move the object anywhere near as much as with on-axis scanning.
The rotating x-ray detector 16 comprises a fluorescent screen 40, a first mirror 42, a second mirror 44, and a turntable 46. The turntable 46 is positioned adjacent the object 14 on the side of the object 14 opposite the x-ray source 12. A camera 56 is positioned opposite the mirror 44 for capturing images reflected into the mirrors 42, 44 from the fluorescent screen 40. The camera 56 may comprise a low light level, closed circuit television camera that produces a video image of the x-ray image formed on the fluorescent screen 40. The camera 56 may be, for example, connected to a video terminal 57 so that a user may observe the image appearing on the detector 40. The camera 56 may also be connected to the image analysis system 15.
In operation, x-rays 34 produced by the x-ray source 12 illuminate and penetrate regions of the object 14 and are intercepted by the screen 40 of detector 16. Synchronous rotation of the x-ray source 12 and detector 16 about the axis 50 causes an x-ray image of a plane within the object 14 to be formed on the detector 16. Although the axis of rotation 50 illustrated in FIG. 1 is the common axis of rotation for both the source 12 and detector 16, as stated above, these axes of rotation are not collinear in an off-axis system, but rather, are parallel to one another. The electron beam 30 is emitted from the electron gun 18 and travels in a region between the electrodes 20 and steering coils 60, 62. The steering coils 60, 62 are separate X and Y electromagnetic deflection coils that deflect the electron beam 30 discharged from the electron gun 18 in the X and Y directions, respectively.! Electrical current flowing in the coils creates a magnetic field that interacts with the electron beam 30, thereby causing the beam 30 to be deflected. The configuration of the x-ray spot pattern on the target 24 depends on where the beam 30 strikes the target 24, which depends on the manner in which the beam 30 is steered. Electrostatic deflection techniques could also be used to deflect the electron beam 30 in the desired directions.
A lookup table (LUT) 63 outputs voltage signals that are applied to the X and Y deflection coils 60, 62 to cause the electron beam spot 32 (FIG. 2) to rotate, thus producing a circular spot pattern on the surface of the target anode 24. The values stored in the LUT 63 are predetermined using a calibration technique that correlates the position of the turntable 46 (i.e., the rotational position of the detector 16 and the position of the x-ray beam spot 32). The values stored in the LUT 63 correspond to the rotational positions of the turntable 46. The turntable outputs electrical signals as it rotates that correspond to its rotational position. Once calibration has been performed using these electrical signals, the calibrated electrical signals are converted into digital values and stored the LUT 63 at appropriate addresses and off-axis laminography is then performed.
It should be noted that the target anode 24 in the '012 patent is flat. Because the target anode 24 is flat, it is difficult for the system 10 to focus on oblique objects, or oblique portions of otherwise planar objects. The term “oblique”, as that term is used herein, is intended to indicate a position that is not in the X, Y plane represented by the X, Y and Z axes shown in FIG. 1. The term “planar”, as that term is used herein, is intended to denote a position that is in the X, Y plane. Thus, the flat target anode 24 shown in FIG. 1 is in the X, Y plane.
Some objects, such as printed circuit boards, for example, are warped or bowed in some fashion, and therefore are oblique or have portions or features that are oblique. It would be desirable to provide an off-axis scanning system that traces circular scan patterns on a target anode in a manner similar to the manner in which the system 10 of the '012 patent operates, but that has the ability not only to precisely image planar objects, but that is also well suited for imaging oblique objects. A need exists for such a system because many objects that laminography techniques are used to inspect are oblique or have portions that are oblique. In addition, such a system could increase the types of objects that can be precisely imaged using laminography.